This invention is related to the field of intensity controls, or dimmers, for fluorescent lamps. More particularly, it is related to intensity controls in which a triac regulates the current flow through the fluorescent lamp.
It is well known to use triacs or controlled rectifiers in dimmer circuits for fluorescent lamps. Fluorescent lamps cannot effectively be controlled by a simple variable resistance, as with incandescent lamps, because fluorescent lamps are closely matched with their associated ballast in order to maintain an effective power factor. Introducing a variable resistance into the circuit would provide low power factors at certain intensity settings, thereby diminishing the efficiency of the circuit.
Accordingly, it has become common practice in the art of controlling fluorescent lamps to use triacs as the main control element. A control circuit is provided to the triac in order to provide proper triac switching in phase with the input power signal. One of the more efficient types of a control circuit is a phase-locked loop. An example of such a circuit is disclosed in U.S. Pat. No. 4,039,897.
The present invention also uses a triac control with a phase-locked loop control circuit. The present control circuit is in many ways similar to the control circuit for use with a.c. induction motors disclosed in U.S. Pat. No. 4,052,648, and produced under license by the present inventor, and herein incorporated by reference.